111 5. Geophysical methods 5.6 Conclusions The Hydrogeological Experimental Site (HES) of Poitiers, which was developed to conduct long-term monitoring and experiments on water and mass transfer processes, has been the site of hydrogeological and geophysical surveys as part of this study. The chapter provides an overview of various field experiments based on seismic and acoustic methods and shows their relative contribution to knowledge of the experimental site. A 3D seismic reflection and refraction survey was carried out. The 3D seismic data was processed to obtain both a structural model in depth (VSP data being used to define a time to depth conversion law), and the distribution of seismic velocities in the reservoir, using an inversion process with acoustic velocity logs as constraints. The result was a high-resolution 3D seismic block, with nearly horizontal stratification and strong lateral variations of velocity. This made it possible to identify three high-porosity, presumably water-producing layers, at depths of 35 to 40 m, 85 to 87 m, and 110 to 115 m. The 85 to 87 m deep layer is the most porous, with porosities of over 30%, which represents the karstic part of the reservoir. To confirm the presence of karstic levels, borehole data were recorded in several boreholes in different frequency bandwidths. A methodology was developed to detect flow using both ambient noise measurement and VSP data. In low frequency band (10–150 Hz), VSP data were recorded with a hydrophone sensor, known to be sensitive to Stoneley waves that are currently used to detect discontinuities such as fractures or faults. Conversion of the down-going P-waves into Stoneley waves was observed at the level of the karstic bodies. This phenomenon occurs in highly permeable formations. Analysis of the ambient noise shows that variations of its characteristics (spectral variance) are correlated with the conversion level of P-waves into Stoneley waves. A VSP attribute, called VSP flow index, was calculated to detect both karstic levels and flows. In high frequency bandwidth (1–20 kHz), full waveform acoustic data were recorded, both to locally validate the results obtained by the seismic methods (3D and VSP) and to evaluate the potential of the acoustic method to detect karstic bodies with a very high resolution. An acoustic attribute, called Karstic Index, was calculated to detect karstic levels. Hydrogeological sites are currently investigated primarily by hydrogeological methods. In this study, we have shown how electrical logging (long normal logs), seismic and acoustic methods (3D seismic, full waveform acoustic logging, and VSP) can contribute to the understanding and description of karstic formations at different scales. Both hydrogeological and geophysical investigations contribute to hydrogeological model building and site description. Figure 19 shows the benefit of combining the two types of investigation. The figure is a synthesis of the methods developed to detect karstic bodies and to quantify flows. The synthesis gathers data from acoustic
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